The oldest known method in the art of utility pole construction is the use of wooden poles, such as those commonly used for telephone lines. However, many modern utility pole uses require longer lengths than are practical, or even possible, with wood, and wood poles are highly susceptible to rot, insect infestation, bird attack and high winds. Additionally, construction of wooden poles requires that the pole be of one piece of uncut wood which creates difficulties in transporting and erecting long poles. To overcome the shortcomings of wood utility poles, solid concrete poles are often used. Concrete utility poles, however, are expensive to produce, heavy and require special heavy duty equipment to load, transport, unload and install. Further, the greater weight of concrete poles precludes the use of very long poles. Metal poles have long served as an alternative to wood and concrete utility poles. Metal poles are relatively strong and capable of being constructed in sections for ease of transportation and erection. The widespread use of metal poles however has been limited since steel poles are expensive to produce and aluminum alloy utility poles do not have sufficient strength to be used in high lateral force environments.
More recently, the concept of using ductile iron for utility pole construction has been suggested. It is believed that ductile iron utility poles will provide a virtually maintenance free, extremely long life, low cost utility pole. An exemplary ductile iron utility pole is disclosed in U.S. Patent Application Publication No. 2008/0023172 A2 to Waugh. The pole is a centrifugally cast utility pole having a tapered exterior and a substantially uniform wall thickness along the long axis of the pole. Additionally, U.S. Pat. No. 5,784,851 to Waugh discloses a hollow, centrifugally cast, utility pole having tapered external linear dimensions. The pole is formed utilizing conventional centrifugal casting methods wherein a tapered mold is used to impart a tapered shape to the pole. The use of the tapered mold during the casting operation also provides for a gradually increasing pole wall thickness along the entire length of the pole from the top of the pole to its butt.
Metal utility poles such as the centrifugally cast poles described above are fabricated to provide for a press-fit, slip joint at the butt of the poles which allows the poles to be interconnected with other similarly cast poles for extended height. Such joints however possess several shortcomings. For a conventional press-fit, slip joint between hollow metal pole members, the standard joint length L is about 1.5 times as large as the inner diameter D of the butt of the pole (L/D=1.5). At this joint length, the strength of the joint between pole members is lower than the strength of the remainder of the poles. Further, in order to make the strength of the joint equal to that of the main body of the poles, the joint length must be increased to about twice as large as the inner diameter of the pole (L/D=2.0). However, if the joint length is made about twice as large, the weight of the metal at the joint increases by about 25 percent. Consequently, the cost of manufacturing the pole increases. Lastly, a joint length where L/D=1.5 equates to a large overlap between interconnected pole members, and thus a large amount of wasted utility pole length.